448 
PALAEONTOLOGY OP NEW-YORK. 
The hinge-line is often much extended, and in the dorsal valve nearly 
straight to the cardinal angles. There is sometimes the appearance of a 
false area on the ventral valve, somewhat similar to Atrypa, the margin 
being thickened and grooved; but this does not appear to be a characte¬ 
ristic or constant feature. 
The specimens of the interior, which have fallen under my observation, usually 
preserve only the short crural processes; and it is in one specimen alone, which is 
partially filled with crystalline matter, that distinct cavities can be seen correspond¬ 
ing with what I have described. The crystalline matter was first deposited upon 
these internal organs, which have subsequently almost entirely decomposed, leaving 
in the cavities fragments of the substance, showing the original form of the crura 
and appendages. 
The shells of this form have been described as Terebratula, and more recently 
have been included with Rhynchonella, from which they differ conspicuously in 
the great inequality of the valves and the extension of the hinge-line, as well as in 
their internal structure. 
I have heretofore referred to this genus the Leptoccelia ( Atrypa ) disparilis of the 
Niagara group, the Terebratula lepida of Goldfuss, the T. sublepida and T. duboisi of 
MM. Murchison, de Verneuil and Keyserling ( Geology of Russia and the Ural 
Mountains). The L. concava and L. imbricata of the Lower Helderberg group are 
analogous forms, which have been referred to this genus. The Atrypa hemispherica 
of Murchison is apparently a characteristic form of this genus, as well as the A. 
planoconvexa , both of the Clinton group*. The internal structure, however, has been 
determined from the species of the Oriskany sandstone and Lower Helderberg 
specimens, but mainly from the former. 
In its hinge structure it approaches Tropidoleptus; but the hinge-line of the 
dorsal valve is never so far produced, while the beak is much more extended. In 
Tropidoleptus there is a distinct linear area, and the teeth and sockets are strongly 
crenulated; the form and character of the muscular impressions are different, and 
the structure of the shell is punctate. 
Geological range. The genus begins its existence as low down in the system as 
the Clinton group, and extends through all the members of the series to the lime¬ 
stones of the Upper Helderberg, and perhaps above that point. 
* The last-named species presents some differences in the hinge structure, which may prove of sufficient 
importance to unite the lower group of species under another generic designation. 
